Summary In hisProgress and its Problems, Laudan dismisses the problem of incommensurability in science by endorsing two general assertions. The first claims there are actually no incommensurable pairs of theories or research traditions; the second maintains that his problem-solving model of scientific progress would be able rationally to appraise even incommensurable pairs of theories or traditions (are compare them for their progressiveness). I argue here that Laudan fails to provide a plausible defence of either thesis, and that this creates some problems for his general approach